Heat treatment of magnesium alloys



Patented Apr. 7, 1936 UNITED STATES HEAT F MAGNESIUM ALLOYS Adolf Beck, Bitterfeld, Germany, assignor, by mesne assignments, to Magnesium Development Corporation, a corporation of Delaware No Drawing. Application December 21, 1934,

gggal No. 758,634. In Germany December 23, p

6 Claims.

This invention relates to saline heating baths for use in the heat treatment of light metals, and particularly of magnesium alloys.

No salts have hitherto been proposed for us in the production of molten saline baths for the heat treatment of light metals containing magnesium at temperatures exceeding about 300 C. which could be relied upon, with certaintyfnot to react with the magnesium at these comparatively elevated temperatures. It has, it is true,

already been proposed to anneal'magnesium alloys in baths of molten potassium nitrate or pyrosuiphate, and it was thought that the occurrence of a reaction between the saline material and the magnesium could be prevented by the addition of The employment of the fused anhydrous alkali bichromates themselves as the basis of molten 25 saline baths has, however, beenhitherto shunned,

because it was considered that if these powerfully oxidizing salts form a substantial, or even the sole component of the saline bath, and are therefore brought into contact with the reactive metal 30 at high temperatures, a violent reaction with the magnesium alloys must inevitably result. Moreover, not withstanding its relatively low melting point, even sodium bichromate is unsuitable for use within the full temperature range coming under consideration for the heat treat- -ment of such light-metal alloys.

Contrary to all expectation, experiments have now shown that the fused anhydrous bichromates-even though they be present in substantial proportions or tom the sole component of thesaline baths-do not react with magnesium and its alloys up to temperatures above the solidus point of the alloys, and that, by the employment of potassiumand sodium bichroma'te 45 .jointly in suitable relative proportions-preferably 3 parts of sodium bichromate 1 part of potassium bichromate-the'point oi solidification of the sslinemixture can be lowered to such an extent that the mixtures are still in the molten condition, and may therefore be employed at temperatures of as low as 275 C.--which is sufficient for the purposes under consideration.-

At the same time, however, the employment of the fused anhydrous bichromates as bath 55 liquid when applied to aluminium base alloys containing minor percentages of magnesium affords the advantage of also assuring the increased resistance to corrosion previously observed in the treatment of alloys having a high percentage'of magnesium with aqueous solutions 5 of bichromates. In particular, it has been ascertained that also workpieces composed of alloys of the duralumin type which thus contain only comparatively low percentages of magneslum (not more than about 2 percent) undergo 10 such an extensive increase in their capacity of resisting corrosion, when molten saline baths in accordance with the invention are employed, that the hitherto customary anodic oxidation treatment is frequently rendered superfluous. It is 5 believed that this increased resistance to corrosion is due to the formation, on the surface of the metal workpieces under treatment, of a tenuous, but cohesive and strongly adhering skin of reaction products of magnesium withthe bichromate compounds.

Whereas aluminium-base alloys containing magnesium sink in the molten saline baths of the invention, alloys in which magnesium is the basis w component float on the surface, owing to their substantially lower specific gravityand must therefore be submerged in the molten saline bath by the application of suitable means during the treatment.

What I claim is:--

l. A process for the heat-treatment of lightmetal alloys containing magnesium, which comprises submerging said alloys in a bath, substantially consisting of at least one molten anhydrous compound of the group consisting of sodium bichromate and potassium bichromate.

2. A process for the heat-treatment of light- 1 metal alloys containing magnesium, which comprises submerging said-alloys in a bath substantially consisting of a mixture of sodium bichromateand potassium bichromate in an anhydrous form.

3. A process for the heat-treatment of lightmetal alloys containing magnesium, which comprises submerging said alloys in a bath substantially consisting of about three parts of anhydrous bichromate and about one part of 'anhyof light-metal alloys containing magnesium at temperatures exceeding about 275 C., which bath substantially consists of a mixture of sodium bichromate and potassium bichromate in an anhy- 5- drous form. I 6. A molten saline bath for the heat-treatment of light-metal alloys containing magnesium at temperatuwes exceeding about 275 C., which bath substantially consists of about three parts of anhydrous sodium bichromate and about one part of anhydrous potassium bichromate.

ADQLF BECK. 

